In this context the expression classic HVDC comprises an HVDC system containing a current source converter (CSC) containing non-extinguishable semiconducting elements such as a thyristor. The expression light HVDC comprises in this context an HVDC system containing a VSC. For both the classic HVDC and the light HVDC AC filters are necessary to reduce the amount of harmonics injected into the connected AC network. Filters are typically designed as resonance circuit tuned to the relevant order of harmonics. The main components of a filter are capacitors, reactors and resistors. Filters are commonly built out-doors as a part of a sub-station switchyard.
Filters in connection with HVDC systems are known to radiate sound to the environment. Sometimes this radiation may be decreased by means attached directly to the radiating equipment itself. Examples of such means are screens and sound absorbing material. These measures often come into conflict with the need for cooling of the equipment. Such measures also increase the weight of the equipment and may be difficult to apply at high voltage potentials. Therefore, and especially in connection with HVDC Light it is known to house the filter inside a building. An indoor location of a filter offers the advantages of reducing both the audible and the electromagnetic emitted noise. Further an indoor filter equipment is protected from airborne pollution.
When the system voltage is increased the size of a filter increases. For capacitor means it is known to arrange a plurality of capacitor element in a stack. When connected the voltage level increases with the height of the capacitor stack. Typically the height of the main capacitor stack together with the necessary air clearance determines the height of the building. The other components which have a lesser height mostly contribute to the footprint area. The footprint area is the necessary area for spreading out the filter equipment including the necessary air clearance between filter components. When housing a normally designed filter indoor the space efficiency of the building volume is very poor. Only the capacitor uses the height while the other components, together with the air clearance, mainly determine the length and width. Hence, a large volume of the building is not used.
In an indoor solution it is also beneficial to have the main feeder bus indoors; otherwise the reduction of radio interference and the purpose of protection from pollution are not achieved.
It is known to mitigate audible noise by specially designed components which are expensive. It is further known to use material and equipment to make the source of sound less emitting or by screening of individual components. Emission of radio frequencies is known to be reduced by installation of dedicated additional filters. The pollution problem is known to be solved by longer creepage distances for the insulators or by coating for hydrofobicity.
For systems with lower voltage (and power) these problems are less significant.